1. Field
This present invention relates to a passive containment cooling system for a nuclear reactor power plant and more specifically to a passive containment air cooling system that relies on natural circulation of air over the surface of a metal containment.
2. Related Art
Nuclear power has played an important part in the generation of electricity since the 1950s and has advantages over thermal electric and hydraulic power plants. The generation of electricity by nuclear power is accomplished by the nuclear fission of radioactive materials. Due to the volatility of the nuclear reactions, nuclear power plants are required by practice to be designed in such a manner that the health and safety of the public is assured even for the most adverse accident that can be postulated. For plants utilizing water as a coolant, the most adverse accident is considered to be a double ended break of the largest pipe in the reactor cooling system and is termed a loss of coolant accident (LOCA).
For accident protection, these plants utilize containment systems that are designed to physically contain water, steam and any entrained fission products that may escape from the reactor cooling system. The containment system is normally considered to encompass all structures, systems and devices that provide ultimate reliability and complete protection for any accident that may occur. Engineered safeguard systems are specifically designed to mitigate the consequences of an accident. Basically, the design goal of a containment system is that no radioactive material escapes from the nuclear power plant in the event of an accident so that the lives of the surrounding populous are not endangered.
Recently, reactor manufacturers have offered passive plant designs, i.e., plants that will shut down in the event of an accident without the operator intervention or off-site power. Westinghouse Electric Company LLC offers the AP1000 passive plant design that employs a passive containment cooling system that uses a large steel shell. The containment cooling system suppresses the rise in pressure that will likely occur within the containment in the unlikely event of a loss of coolant accident. The passive containment cooling system is an engineered safety feature system. Its objective is to reduce the containment temperature and pressure, following a loss of coolant accident or steam line break accident inside the containment, by removing thermal energy from the containment atmosphere. The passive containment cooling system also serves as a means of transferring heat for other events resulting in a significant increase in containment pressure and temperature. The passive containment cooling system also limits release of radioactivity (post accident) by reducing the pressure differential between the containment atmosphere and the external environment, thereby diminishing the driving force for leakage of fission products from the containment to the atmosphere. The passive containment cooling system also provides a source of make-up water to the spent fuel pool cooling water. To achieve the foregoing objectives, the containment building is made of steel to provide efficient heat transfer from within to outside of the containment. During normal operation, heat is removed from the containment vessel by continuous natural circulation of air. During an accident, however, more heat removal is required and air cooling is supplemented by evaporation of water, provided by a passive containment cooling system water storage tank and gravity feed.
An AP1000 containment system 10 is schematically illustrated in FIG. 1, surrounding an AP1000 reactor system including a reactor vessel 12, steam generator 14, pressurizer 16 and main coolant circulation pump 18; all connected by the piping 20. The containment system 10 in part comprises a steel dome containment vessel enclosure 22 surrounded by a concrete shield building 24 which provides structural protection for the steel dome containment vessel 22.
The major components of the passive containment cooling system are a passive containment cooling water storage tank 26, an air baffle 28, air inlet 30, air exhaust 32 and water distribution system 34. The passive containment cooling water storage tank 26 is incorporated into the shield building structure 24, above the steel dome containment vessel 22. An air baffle 28 located between the steel dome containment vessel 22 and the concrete shield building 24 defines the cooling air flow path which enters through an opening in the shield building 24 at an elevation approximately at the top of the steel dome containment vessel 22. After entering the shield building 24, the air path travels down one side of the air baffle 28 and reverses direction around the air baffle at an elevation adjacent the lower portion of the steel dome containment vessel. The air path then flows up between the baffle and the steel dome containment vessel 22 and exits at the exhaust opening 32 in the top of the shield building 24. The exhaust opening 32 is surrounded by the passive containment cooling water storage tank 26.
In the unlikely event of an accident, the passive containment cooling system provides water that drains by gravity from the passive containment cooling water storage tank 26 and forms a film over the steel dome containment vessel 22. The water film evaporates thus removing heat from the containment building 22.
The passive containment cooling system is capable of removing sufficient thermal energy, including subsequent decay heat, from the containment atmosphere following a Design Basis event resulting in containment pressurization such that the containment pressure remains below the design value with no operator action required for at least 72 hours.
The air flow path that is formed between the shield building 24, which surrounds the steel dome containment vessel 22, and the air baffle 28 results in the natural circulation of air upward along the containment vessel's outside steel surface. This natural circulation of air is driven by buoyant forces when the flowing air is heated by the containment steel surface and when the air is heated by and evaporates water that is applied to the containment surface. The flowing air also enhances the evaporation that occurs from the water surface. In the event of an accident, the convective heat transfer to the air by the heated containment steel surface only accounts for a small portion of the total heat transfer that is required, such total heat transfer being primarily accomplished by the evaporation of water from the wetted areas of the containment steel surface, which cools the water on the surface which then cools the containment steel, which then cools the inside containment atmosphere and condenses steam within the containment.
Water is continuously applied via gravity from the passive containment cooling water storage tank 26 to the containment vessel steel surface 22 for the first seventy two hours following a Design Basis event. The application of water to the containment vessel steel surface 22 enhances heat transfer through the vessel and aids in condensing the steam within the containment, therefore also limiting the pressure increase within the containment. After the first seventy two hours, active onsite pumping methods will provide makeup water to the passive containment cooling water storage tank 26 for at least an additional four days. Additional onsite and offsite water sources and pumping methods continue to provide makeup water to the passive containment cooling water storage tank 26 after seven days.
It is an object of this invention to enable air cooling alone of the containment vessel to provide sufficient decay heat removal to maintain acceptably low containment pressure after the initial three days.
Furthermore, it is an object of this invention to enable air cooling of the containment vessel to provide such sufficient decay heat removal with no reliance on active components, operator actions, or non-safety onsite or offsite water supplies.
Additionally, it is an object of this invention to provide sufficient air cooling of the containment vessel that will enable a reduction in the size of the passive containment cooling water storage tank that is required.